1. Field of the Invention
This invention is related to the monolithic integration of optically-pumped and electrically-injected III-nitride light-emitting devices.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets [Ref. x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.) This invention describes a structure for improving the performance of III-nitride light-emitting devices. The term “III-nitrides” refers to any alloy composition of the (Ga,Al,In,B)N semiconductors having the formula GawAlxInyBzN where 0≤w≤1, 0≤x≤1, 0≤y≤1, and 0≤z≤1.
The usefulness of III-nitrides has been well established for the fabrication of visible and ultraviolet opto-electronic devices and high power electronic devices. Current state-of-the-art III-nitride thin films, heterostructures, and devices are grown along the polar [0001] c-axis. The total polarization of such films consists of spontaneous and piezoelectric polarization contributions, both of which originate from the single polar [0001] c-axis of the wurtzite III-nitride crystal structure. When III-nitride heterostructures are grown pseudomorphically, polarization discontinuities are formed at surfaces and interfaces within the crystal. These discontinuities lead to the accumulation or depletion of carriers at surfaces and interfaces, which in turn produce electric fields. Since the alignment of these polarization-induced electric fields coincides with the typical polar [0001] c-plane growth direction of III-nitride thin films and heterostructures, these fields have the effect of “tilting” the energy bands of III-nitride devices.
In c-plane wurtzite III-nitride quantum wells, the “tilted” energy bands spatially separate the electron and hole wavefunctions. This spatial charge separation reduces the oscillator strength of radiative transitions and red-shifts the emission wavelength. These effects are manifestations of the quantum confined Stark effect (QCSE) and have been thoroughly analyzed for III-nitride quantum wells (QWs). [Refs. 4-7] Additionally, the large polarization-induced electric fields can be partially screened by injected carriers, [Ref 8] making the emission characteristics difficult to engineer accurately.
One approach to decreasing polarization effects in III-nitride devices is to grow the devices on nonpolar planes of the crystal. These include the {11-20} planes, known collectively as a-planes, and the {10-10} planes, known collectively as m-planes, wherein the use of braces, { }, denotes a set of symmetry-equivalent planes, which are represented by the use of parentheses, ( ). Such planes contain equal numbers of gallium and nitrogen atoms per plane and are charge-neutral. Subsequent nonpolar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction.
Another approach to reducing polarization effects in III-nitride devices is to grow the devices on semipolar planes of the crystal. The term “semipolar plane” can be used to refer to any plane that cannot be classified as c-plane, a-plane, or m-plane. In crystallographic terms, a semipolar plane would be any plane that has at least two nonzero h, i, or k Miller indices and a nonzero 1 Miller index. Subsequent semipolar layers are equivalent to one another, so the bulk crystal will have reduced polarization along the growth direction.
Despite these inherent advantages, challenges still remain for nonpolar and semipolar device growth. For example, when green III-nitride LEDs and LDs are grown with active regions with high Indium contents, the active region can form extended defects and can easily be degraded by subsequent high temperature growth steps. [Refs. 1,2] In particular, the growth of subsequent p-type layers can be especially damaging, as these layers usually need to be grown at elevated growth temperatures to ensure adequate p-type conduction. [Ref 3]
Thus, there is a need in the art for improved methods of fabricating III-nitride light emitting diodes. The present invention satisfies this need.